With computerization of business, regulations, and the like, there are increasing demands for storage media that store electronic mails and fully electronic documents and papers.
The storage media storing electronic data are predominantly configured to be capable of storing large-capacity data with quick responsiveness. As such storage media, hard disk drives (HDD) and the like are known.
Further, in addition to the hard disk drives and the like, with a view to reducing the electric power consumption of devices (storage devices) in which storage media are mounted, there are increasingly used SSD (solid state drives) that use semiconductor storage elements, such as flash memories, which can achieve increased write speed and reduced electric power consumption of disks.
It is known that the storage devices are constructed by taking redundancy into account (see Japanese Laid-Open Patent Publication No. 2008-52313). When the storage devices are constructed by taking redundancy into account, modules for controlling the storage devices, disks, and SSDs are made at least duplex.
See Japanese Laid-Open Patent Publication No. 2009-163647, and Japanese Laid-Open Patent Publication No. 2008-52313.
When a write access to an SSD occurs with high frequency, the SSD can reach an upper limit of the number of times of rewriting that can be executed, earlier than the hard disk drives, due to the structure of the semiconductor elements thereof.
Although the above description has been given of the SSDs, the other nonvolatile semiconductor memories also suffer from the same problems.